EMI suppression coding

ABSTRACT

EMI suppression technique that pseudorandomizes the positive or negative going transitions that are used to represent one binary state in a transmitted digital signal such as in MLT-3 data format. By pseudorandomizing the selection of these transitions, substantial spreading of the spectral energy occurs. Descrambling need not occur since each transition is recognized as the encoded binary state. Thus, the data sender can encode the data and suppress EMI without any changes to the receiver&#39;s equipment.

This is a continuation of application Ser. No. 07/964,508, filed Oct.21, 1992, now U.S. Pat. No. 5,283,807.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of electromagnetic interference (EMI)suppression and digital coding techniques used to suppress theinterference.

2. Prior Art

Some digital data encoding techniques uses three signal levels to encodetwo digital states. The bipolar format used in "T1" uses a zero signallevel to represent a binary 0 and a positive or negative level torepresent a binary 1. The positive and negative levels are alternated toensure a zero DC level independent of the data which is encoded. Theproperties of this carrier system as well as others, are discussed inDigital Transmission Systems, by David R. Smith, published by VanNorstrand, Reinhold Company (1985) particularly in Chapter 5 entitled:"Baseband Transmission".

In a high bit rate bipolar system, when certain code sequences aretransmitted such as a string of binary ones, electromagneticinterference (EMI) occurs which may exceed set regulatory levels. Thisis particularly true since EMI is often measured over relatively narrowfrequency bands.

One technique to reduce EMI is to scramble the data prior totransmission and descramble it at the receiver. This scrambling can takethe form of exclusively oring the data stream with a pseudorandom codesequence. To descramble, the received signals is exclusively ored withthe same pseudorandom code sequence used for scrambling. This of courserequires synchronization between the two pseudorandom code sequences.This is sometimes accomplished by using a predetermined algorithm whichis synchronized at the transmitter and receiver.

As will be seen, the present invention provides EMI suppressionparticularly for an encoding mechanism employing three distinct signalcharacteristics, such as three levels, by randomizing the transmittedsignal. However, as will be seen this randomizing does not alter thedecoded data signal. Consequently, no descrambling is needed.

SUMMARY OF THE INVENTION

A method and apparatus for suppressing electromagnetic interference(EMI) is described particularly for a system which communicates data byencoding one data state with two different signal characteristics suchas two different signal levels. With the present invention, theselection between the two different signal characteristics for the onedata state is pseudorandom as opposed to being alternated. This spreadsthe frequency spectrum thereby reducing measured EMI.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plurality of waveforms used to describe the prior art andthe present invention.

FIG. 2 is an electrical schematic showing one embodiment of the presentinvention.

FIG. 3 is an electrical schematic of a prior art encoder.

FIG. 4 is an electrical schematic of a prior art decoder.

FIG. 5 illustrates a portion of the encoder of FIG. 3 with theimprovement of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Electromagnetic interference (EMI) suppression method and apparatus isdescribed for digitally encoded data. In the following description,numerous well-known circuits and techniques are not described in detailin order not to unnecessarily obscure the present invention. In otherinstances, specific waveforms and circuits are provided in order toprovide a thorough understanding of the present invention, it will beobvious, however, to one skilled in the art that the present inventionmay be practiced without these specific details.

Referring first to FIG. 1, a bit stream 10 of digital data isrepresented by zeros and ones. The waveforms below this bit streamillustrate various methods of encoding the data represented by the bitstream 10. The bit stream 10 may have been previously encoded with, forexample, the 4 to 5 FDDI coding.

Waveform 11 represents the data stream 10 using the well-knownnonreturn-to-zero (NRZ) coding, more specifically, NRZ(L). This waveformhas two levels: the lower level indicating a binary 0 and the upperlevel a binary 1.

Waveform 12 illustrates the bit stream encoded in another well-knownmanner where three discrete signal levels are used: a zero level, apositive level and a negative level. A binary 1 is represented by eitherthe positive or negative levels, the binary 0 is represented by the zerolevel. With this encoding a transition occurs with each binary 1 and thepositive and negative levels are alternated to avoid a DC component inthe signal. By way of example, the first two binary ones of the bitstream 10 cause a transition to the positive level 13 and then to thenegative level 14. The following binary 0 in the bit stream causes atransition to the zero level 15.

The waveform 16 simply represents a clocking signal with a period equalto the bit period shown for the various waveforms of FIG. 1. When thisclocking signal is anded with the NRZ(L) signal a return-to-zero (RZ)encoding results. The waveform 17 is the widely used "T1" carrier systemwhich results from anding waveforms 11 and 16 and alternating thetransitions such as was done for waveform 12.

The NRZ(l) waveform 18, also well-known, can be generated directly fromthe NRZ(L) signal along with the clocking signal 16 as will be shown inconjunction with FIG. 3.

The MLT-3 waveform 21 also is a well-known encoding technique, itsgeneration is described in conjunction with FIG. 3.

Waveform 19 represents a pseudorandom code sequence which may begenerated with any one of a plurality of well-known pseudorandomgenerators. As will be seen, this pseudorandom code is used to select orcontrol, for example, whether a binary 1 of a randomized representationof waveform 17 will be represented by a positive or negative level.

As mentioned earlier, the prior art encoding represented by waveforms 12and 17 alternates between positive and negative transitions to representa binary 1. With the present invention, the pseudorandom code ofwaveform 19 controls the selection of either the positive or negativelevels. For instance, while the positive level 23 represents the firstbinary 1 of the bit stream 10 of waveform 17, a negative level 26(waveform 20) is used as taught by the present invention. Thedetermination to use a negative level is governed by the low level ofthe pseudorandom code sequence 19 at time 28. The next binary 1 of thebit stream 10 is represented by a positive level 27 since at time 29 thepseudorandom signal 19 is high. When the next binary 1 occurs in the bitstream 10 (time 30), a negative level 31 is used to represent thisbinary 1 since at time 30 the waveform 19 is low. As will be discussedlater, randomizing the selection of the positive and negativetransitions of the waveform 20, spreads the spectral content of thesignal. This has the effect of reducing the EMI.

In a prior art receiver/decoder each transition from zero is detected asa binary 1 for the waveform 20. The direction of the transition, (i.e.,positive going or negative going) is not significant. Therefore, thereceiver need not know the pseudorandom code sequence used for encodingthe waveform 20. Thus, while the transmitted waveform is scrambled bythe pseudorandom signal, the receiver need not know that this hasoccurred and certainly does not need to know the specific randomizedcode sequence or algorithm used to randomize the transmitted signal.Since the selection between the positive going and negative goingtransitions of waveform 20 are randomized, the net result will be no netDC current.

A circuit for providing the waveform 20 is shown in FIG. 2. Thepseudorandom code generator 40 provides the waveform 19 to the Dterminal of a flip-flop 44. The data represented by the waveform 11 iscoupled by line 41 to one input terminal of an AND gate 42. The clockingsignal 16 is coupled by a line 43 to the other input terminal of the ANDgate 42. (The clocking signal is also used by the generator 40 tosynchronize the output of the generator 40.) The output of AND gate 42is coupled to the clock terminal of the flip-flop 44 and to one inputterminal of each of the AND gates 45 and 46. As would be obvious to oneskilled in the art, the clocking signal supplied to AND gate 42 isdelayed so that any transitions in the output of the generator 40 occurbefore any transitions in the output of AND gate 42. This ensures thatthe pseudorandom code of waveform 19 received by the D terminal offlip-flop 40 transitions and is stable before clocking the flip-flop 40.The other input terminal of AND gate 45 receives the Q output offlip-flop 44 and the other terminal of the AND gate 46 receives the Qoutput of the flip-flop 44. The driver 47 which may be a well-knowncircuit, provides a positive going transition on line 48 when the outputof the gate 45 is high and a negative going transition on line 48 whenthe output of gate 46 is high. When the output of gates 45 and 46 areboth low, as will occur anytime the data on line 41 is low, a 0 levelsignal is provided on line 48. Thus, by coupling the waveforms shown inFIG. 1 to the circuit of FIG. 2, the randomized waveform (shown asrandomized T1) is provided on line 48. For the particular positive andnegative transitions shown in waveform 20, it is assumed that theflip-flop 44 is in one predetermined state upon initialization. This isnot significant since if the flip-flop 44 were in its other state uponinitialization, the waveform 20 would simply be reversed, that is allthe positive pulses would become negative pulses, and all the negativepulses would be positive pulses.

FIGS. 3, 4 and 5 are used to illustrate a prior art coding and decodingcircuit and the change which is made to the encoding circuit toimplement the present invention. When the present invention is used, theprior art decoder/receiver of FIG. 4 may be used without change.

Referring now to the prior art encoder of FIG. 3, it receives the NRZ(L)signal at one input terminal of the AND gate 50; the other inputterminal of this AND gate receives the clocking signal. These signalscorrespond to the waveforms 11 and 16, respectively of FIG. 1. Theoutput of gate 50 is applied to the clock input terminal of flip-flop51. The Q output of flip-flop 51 is applied to the D input of thisflip-flop. The signal on the Q terminal of the flip-flop (line 52) isthe well-known NRZ(I) signal shown as waveform 18 of FIG. 1. This signalis coupled to the clock terminal of the flip-flop 53 and one inputterminal of each of the AND gates 54 and 55. The other input terminal ofthe AND gate 54 is coupled to the Q terminal of flip-flop 53; the Qterminal of this flip-flop is coupled to the other input terminal of theAND gate 55. The driver 56 provides the MLT-3 signal (line 57) shown aswaveform 21 of FIG. 1. Again, as was the case with the driver of FIG. 2,when the output of the AND gate 54 is high the signal on line 57 ishigh; when the output of the AND gate 55 is high, the signal on line 57is low; and, when the outputs of both the AND gates are low, the signalon line 57 is at the zero level. The connection between the Q terminalof the flip-flop 53 and the D terminal of the flip-flop assures that thepositive going and negative going transitions from the zero level of thesignal on line 57 alternate.

The prior art decoder of FIG. 4 receives the signal on line 57 at anequalizer/slicer 60. This well-known circuit provides a thresholdvoltage for the comparators 61 and 62. The MLT-3 signal (waveform 21 ofFIG. 1) is coupled to the negative terminal of the comparator 61; and,the inverse of this signal (through the inverter 63) is coupled to thenegative terminal of the comparator 62. The outputs of the comparator 61and 62 are coupled to the OR gate 64. The output of this OR gate is theNRZ(l) signal which is coupled to one input terminal of exclusive ORgate 66. The NRZ(l) signal after being delayed by one clock time in theflip-flop 65, is coupled to the other input terminal of this exclusiveOR gate. The output of the gate 66 is the NRZ(L) signal (line 67).

To implement the present invention with the prior art circuits of FIGS.3 and 4 requires that the pseudorandom code be coupled to the D terminalof the flip-flop 53 as shown in FIG. 5. That is, by way of example, theoutput of a pseudorandom code generator 40 of FIG. 2 is coupled via aline 70 to the flip-flop 53. Placing the pseudorandom code on the Dterminal of this flip-flop (rather than the Q signal as shown in FIG. 3)causes the negative and positive going transitions of the MLT-3 signalto be randomized. Consequently, the output signal on line 71 willcorrespond to the waveform 22 of FIG. 1 again assuming that theflip-flops are in predetermined states upon initialization. Otherwise areversal of this waveform may occur. As mentioned, this randomizedwaveform may be decoded by the prior art circuit of FIG. 4 withoutchange to the circuit of FIG. 4.

In the prior art, as previously mentioned, data is sometimes scrambledon encoding and descrambled on decoding to spread the spectral energyand thereby reducing EMI. With the present invention, particularly withencoding techniques such as T1 or MLT-3, the benefits of the spectralspreading can be obtained without the complexity associated with thedescrambling.

The present invention may also be used where an algorithm is used togenerate the code represented by the waveform 19 of FIG. 1. Thealgorithm may be used to obtain a predetermined spectral characteristicin the transmitted signal That is, instead of a pseudorandom code, thecode generated using the algorithm can be tailored to achieve somepredetermined spectrum.

For example, if the clock rate for the data being transmitted is 100 MHzand a notch in the EMI spectrum is desired at 35 MHz, the data can beexamined by passing it through a 35 MHz band pass filter to determinethe signal content at 35 MHz. Where the spectral content is found to behigh at the output of the filter, a predetermined bit pattern can beforced into the pseudorandom code to cause the spectral energy at 35 MHzto be shifted to a higher or lower frequency. More specifically,referring to FIG. 3 a delay is added into the signal on line 52 to allowthe NRZ(l) signal to be examined for the 35 MHz component. The filter'sinput is coupled to line 52 and its output is coupled to a codegenerator such as generator 40 of FIG. 2. When the filter's outputindicates a high signal content at 35 MHz, a predetermined code sequenceis applied to the D terminal of the flip-flop 53 to cause the energy atthis frequency to be shifted either to a higher frequency or lowerfrequency.

Thus, a method and apparatus has been described which suppresses EMI byrandomizing the transmitted signal, without requiring descrambling atthe decoder.

I claim:
 1. In a method for communicating binary data where one binarydata state is represented by either one of two different signal levels,and another binary data state is represented by another signal leveldifferent than said two different signal levels, an improvementcomprising the steps of:generating a pseudorandom signal; selectingbetween said two different signal levels as a function of saidpseudorandom signal; and, recovering said binary data without the use ofsaid pseudorandom signal.
 2. The improvement defined by claim 1 whereinsaid pseudorandom signal comprises a stream of pseudorandom digitalsignals.
 3. In a method for communicating digital data where one binarystate is represented by either one of two different signal levels andthe other binary state is represented by a third signal level, differentthan said other two different signal levels, an improvement comprisingthe steps of:providing a pseudorandom signal; selecting between twodifferent signal levels as a function of said pseudorandom signal toidentify said one binary state; and selecting said third signal level toidentify said other binary state.
 4. In a method for communicatingdigital data where one digital state of data is represented by a signalhaving a signal level of approximately zero and the other digital stateof data is represented by alternating between positive and negativesignal levels, an improvement to reduce certain electromagneticinterferences comprising the steps of:using said signal level ofapproximately zero to represent said one digital state; and,pseudorandomly selecting between said positive and negative signals torepresent the other digital state wherein the other digital state isrecovered without the use of said pseudorandom signal.
 5. In a method ofcommunicating a digital signal where alternating between two signallevels is used to signify a binary data state, an improvementcomprising:generating a pseudorandom signal; using said pseudorandomsignal to select between said two signal levels for communicating saidbinary data state: and, recovering said binary data state without usingsaid pseudorandom signal.
 6. In a method of communicating a digitalsignal where alternating between two signal levels is used to signifyone binary data state, an improvement comprising:using a control signalto select between said two signal levels; and, generating said controlsignal based on an algorithm which prevents alternating between said twosignal levels; and, recovering said one binary data state without saidcontrol signal.
 7. The improvement defined by claim 6 wherein saidcontrol signal is selected to provide a predetermined spectral responsein said digital signal.
 8. A communication system comprising:an encodercomprising; an encoder means coupled to receive said digital signal forencoding said digital signal such that one binary state is representedby either one of two different signal levels, a signal generator forgenerating a pseudorandom signal; and, a selector for selecting betweensaid two different signal levels under control of said pseudorandomsignal; a decoder comprising means for recognizing said two differentsignal levels as representing said one binary state without the use ofsaid pseudorandom signal; and, a link coupled between said encoder anddecoder.